1. Field of the Invention
The present invention relates to a liquid crystal display device, and particularly relates to a liquid crystal display device having a liquid crystal display panel of which alignment film is provided with alignment controllability by irradiation of light.
2. Description of the Related Art
Liquid crystal display devices include a TFT substrate having pixel electrodes, thin film transistors (TFT), etc. formed in a matrix; a counter substrate disposed in facing relation to the TFT substrate and having color filters, etc. formed at positions corresponding to the pixel electrodes of the TFT substrate; and liquid crystals put between the TFT substrate and the counter substrate. Images are formed by controlling the light transmittance of liquid crystal molecules of each pixel.
Since the liquid crystal display devices are flat and light in weight, their application use has been extended in various fields, for example, from large-sized display devices such as television sets to mobile phones and DEC (Digital. Still Cameras). The liquid crystal display devices involve a problem about a viewing angle characteristic. The viewing angle characteristic is a phenomenon that luminance or chromaticity changes when the screen is viewed obliquely compared with when it is viewed from the front. An IPS (In Plane Switching) system in which liquid crystal molecules are operated by horizontal electric fields has excellent viewing angle characteristics.
A method of applying an alignment treatment, that is, providing an alignment controllability to an alignment film used in the liquid crystal display device includes a rubbing treatment in the prior art. In the alignment treatment by rubbing, alignment is performed by rubbing an alignment film with a cloth. The alignment treatment also includes optical alignment method of providing the alignment controllability to an alignment film in a contactless manner. The optical alignment is applicable to the IPS system since a pre-tilt angle is not required.
JP-A-2004-206091 discloses a photolytic optical alignment treatment by irradiation of light typically represented by UV-light. JP-A-2004-206091 describes that the photolytic optical alignment can provide the following effect of:    (1) mitigating alignment disturbance due to the complicate stepped structure in a Pixel portion, and    (2) overcoming damage to thin film transistors caused by static electricity in the alignment treatment by rubbing and display failure caused by disturbance of alignment due to the disorder at the tip of the rubbing cloth or dusts, thereby overcoming complexity in the process caused by frequent replacement of the rubbing Cloth required for obtaining uniform alignment Controllability.
JP-A-2010-72011 discloses a configuration in which the upper layer of the alignment film is formed of a film having a large molecular weight and capable of efficient optical alignment while the lower layer of the alignment film is formed of a photoconductive film having a small molecular weight thereby decreasing a residual image.